The above-mentioned type of window unit, widely known, is used above all on large curtain walls for ventilating rooms, for example, but without limiting its scope, on high-rise buildings with extensive use of glazing.
The sash can be moved from a closed position, in which it is in contact with the fixed frame, to an open position in which the sash is slidably rotated, away from the fixed frame, until it is able to reach, where required, a position almost at a right angle to the previous one, and vice versa.
For this type of movement, inserted between the fixed frame and the sash on both sides (that is to say, on two opposite sides), there is a pair of hinge units, each comprising:                a supporting and guiding element integral with the fixed frame;        an arm, one end of which is connected to the sash;        jointed connecting elements inserted between the supporting element and the sash supporting arm, designed to allow the arm to move between the closed position and the open position, and vice versa.        
In particular, these connecting elements usually comprise: a slide which is slidably positioned along the supporting element (forming a connecting point which, for the sake of simplicity, will be referred to as the movable pivot);                a plurality of connecting rods between the slide and the arm, designed to form at least one four-bar linkage, to allow rototranslation of the arm away from and towards the supporting element;        a joint rod between the arm and the element, one end of the rod pivoting at a fixed point of the supporting element (forming a second connecting point, referred to as the lower connecting point).        
Moreover, the supporting and guiding element has a proximal end shaped to form a sort of “roof-style” cover (symmetrical or asymmetrical) in which the upper end of the joint arm is positioned when the sash is in the closed position.
To improve the window unit seal, the end of the arm is usually equipped with an insert shaped to match the configuration of the cover, allowing an interference fit to achieve optimum locking and seal between the sash and the fixed frame.
Said hinge units are widely used and it has been shown that the most delicate or critical zone of these units is that of contact between the upper end of the arm and the supporting element proximal upper covering end, in the closed configuration.
This zone is designed so that, at the end of the sash closing movement, it forms the point of contact between the arm and the supporting element, then is compressed (in particular the end of the arm) during sash closing to give stable closing and a weatherproof seal.
It is therefore obvious that, over time, this point is subjected to significant stresses due precisely to the purpose for which it is intended, that is to say, to stabilise window unit closing (by rubbing contact of the cover with the insert) and to maintain a good weatherproof seal when the window unit is closed.
For this reason, as well as the presence of the insert (usually made of plastic material) at present, the asymmetrical cover element configuration is preferred, because the presence of a longer contact side allows a force to be discharged on this (more extensive) upper angled surface than with a symmetrical configuration: this allows greater stability and an improved seal in the closed configuration, also reducing the hinge unit stress from wear.
However, despite this, the hinge unit structured in this way may have several disadvantages due to the limited flexibility of joint system adjustments both during the assembly step and during hinge unit use: this inflexibility, above all regarding the upper contact zone, may cause incorrect contacts between the parts and, above all, a failure of the weatherproof seal.